1. Field of the Invention
This invention relates to a photoconductive thin film formed on a substrate and containing hydrogen and crystal grains of silicon, which is called microcrystalline silicon, and a photovoltaic device such as a solar cell and a photosensor, making use of the photoconductive thin film.
2. Related Background Art
In recent years, there is a trend toward installation of solar cells on the roofs of houses so that they are connected to general electric power systems to cover the demand for electric power in summer. However, the cost for electricity generation by solar cells is still so high that solar cells have not spread on a large scale.
For making the solar cells low-cost, those employing amorphous silicon thin films in photovoltaic layers are considered advantageous, but have problems that they have a lower photoelectric conversion efficiency (hereinafter often "conversion efficiency") than crystal type solar cells and the conversion efficiency decreases during irradiation by light (hereinafter often "light degradation". Accordingly, almost all publication of researches on solar cells making use of amorphous silicon thin films (hereinafter often "amorphous solar cells") concerns with two points, "how high conversion efficiency be achieved" and "how light degradation be made less occur".
Researches concerning microcrystalline silicon containing hydrogen (.mu.c-Si:H) also are almost all held by those aiming at its application to thin-film transistors and solar cells. In particular, a large number of researches are made on its application as doping layers of solar cells on their light-incident sides, making the most of the features that it has a small absorption coefficiency at short wavelength and that it can enjoy a small activation energy. For example, Japanese Patent Application Laid-Open No. 8-116080 (hereinafter "Publication 1" discloses an attempt to enhance open-circuit voltage by using the .mu.c-Si:H in p-type layers of a solar cell in which amorphous silicon thin films are used in i-type layers.
Making the most of the feature that it has a large absorption coefficient at long wavelength, researches are also made energetically on its use in i-type layers and photovoltaic layers of solar cells. For example, Solid-State Phenomena Vol. 47-78 (1996), pp.607-612, "Determination of Subgap Absorption in .mu.c-Si:H Films by CPM", R. Krankenhagen, et al. (hereinafter "Publication 2") reports an absorption coefficient of .mu.c-Si:H that is measured by the constant photocurrent method (CPM). According to this report, Urbach energy Eu is calculated to be about 120 meV.
Thin Solid Films Vol. 167 (1988), pp.121-127, "Change in the Properties of Glow-Discharge-Deposited Microcrystalline Silicon Films with Thickness", S. C. De, Awati and A. K. Barua (hereinafter "Publication 3") also reports that the properties of microcrystalline silicon thin films produced by plasma CVD change depending on layer thickness.
With an increase in layer thickness, hydrogen content decreases from 47% to up to 6.3%. The value of C1/C2 described later is estimated to be about 2 at a layer thickness of 250 nm or larger. It is also reported that dark conductivity (ad) rises from 1.times.10.sup.-10 up to 9.times.10.sup.-3 (1/.OMEGA..multidot.cm) with an increase in layer thickness. This is presumed to be due to the movement of Fermi level to the vicinity of an energy band end, which has made the activation energy small. Such movement of Fermi level is considered due to the presence of many defect levels (tail state) in the energy band.
More recently, a solar cell is reported which is a single cell comprising an i-type layer formed of .mu.c-Si:H has a high conversion efficiency and may cause less light degradation. This solar cell is spotlighted as a substitute for solar cells whose i-type layers are formed of a-SiGe:H. Such a cell can be free from the light degradation that is peculiar to amorphous silicon thin films such as a-SiGe:H film, and also does not require to use any expensive material gases such as germanium gas (GeH.sub.4).
This .mu.c-Si:H thin film has not so large an absorption coefficient as the a-SiGe:H thin film, but has a possibility of attaining a short-circuit photocurrent (Jsc) comparable to the a-SiGe:H single cell when the i-type layer is formed in a layer thickness of 3 .mu.m or larger. As an example of reports on it, MRS Symposium Proceeding Vol. 420, Amorphous Silicon Technology 1996, pp.3-13, "On the Way Toward High Efficiency Thin-Film Silicon Solar Cells by the Micromorph Concept", J. Meier et al. (hereinafter "Publication 4") reports a solar cell whose i-type layer is formed of microcrystalline silicon.
This solar cell is a solar cell produced by VHF plasma CVD making use of a frequency of 110 MHz, and has achieved a conversion efficiency of 7.7% in a single cell having one p-i-n junction (structure). In addition, this single cell has a great advantage that it is almost free from light degradation. Moreover, an additional p-i-n junction having an amorphous silicon thin film as its i-type layer is superposed on it to produce a stacked cell which has achieved a conversion efficiency of 13.1%.
However, it still has a high rate of light degradation, which does not differ from those of conventional amorphous silicon types. Then, it is also reported that, in the results of infrared (IR) spectroscopy, Si-H bonds are little present in the .mu.c-Si:H thin film. Although no numerical values are reported, the slope of tail state (Urbach energy Eu) estimated from CPM absorption coefficient curves is about 66 meV.
Japan Journal of Applied Physics Vol. 36 (1997), pp.L569-L572, Part 2, No. 5A, "Optical Confinement Effect for below 5 .mu.m Thick Film Poly-Si Solar Cell on Glass Substrate", Kenji Ymamamoto et al., Kaneka Corporation (hereinafter "Publication 5") also reports a single cell having a p-i-n junction formed of poly-Si and .mu.c-Si which has achieved a conversion efficiency of 9.8%.
The i-type layer of this cell has a layer thickness of 3.5 .mu.m, which is small for a poly-Si single cell, but has a fairly high short-circuit photocurrent (Jsc) of 26 mA/cm.sup.2. Moreover, an additional p-i-n junction having an amorphous silicon thin film as its i-type layer is superposed on it to produce a stacked cell which has achieved a conversion efficiency of 12.8%.
Japanese Patent Application Laid-Open No. 7-94766 (hereinafter "Publication 6") also reports a solar cell constituted of SiN/p.sup.+ poly-Si/p poly-Si/n.sup.+ poly-Si/Al on a glass substrate, having achieved a conversion efficiency of as high as 10.1% at maximum. The solar cell reported in this patent publication has a feature that the p.sup.+ poly-Si has orientation of (100)- (111)- and (110)- planes. However, its thickness for forming a junction is fairly as large as 10 .mu.m, and film-forming temperature and annealing temperature are also fairly as high as 500.degree. C. to 700.degree. C. Because of such high film-forming temperature and annealing temperature, the hydrogen content in the film is presumed to be less than 1%.
Now, the above prior art has the following problems.
In Publication 1, the i-type layer is an amorphous silicon thin film (a-Si:H, a-SiGe:H or a-SiC:H), and hence the light degradation is unavoidable.
In Publication 2, the Urbach energy is as large as about 120 meV, and hence defect levels are considered present in a large number, thus such a film is by no means usable in photovoltaic devices.
In Publication 3, too, defect levels are considered present in a large number, and such a film is by no means usable in photovoltaic devices.
In Publication 4, the film has few Si--H bonds. Also, its Urbach energy is estimated to be about 66 meV, and the cell has a little low conversion efficiency of 7.7%.
In Publication 5, the cell has a high conversion efficiency of 9.8%, but the poly-Si:H thin film requires to have a layer thickness of about 3.5 .mu.m. This is industrially disadvantageous compared with amorphous silicon thin films. Also, since the thin film is formed at a temperature of 550.degree. C. at maximum, hydrogen is considered to be little contained. Moreover, because of a high temperature process, the substrate used is necessarily limited to glass.
In Publication 6, too, the cell has a high conversion efficiency of 10.1%, but the thickness for forming a junction must be as large as 10 .mu.m. Similarly, since the thin film is formed at a temperature of 550 to 700.degree. C., hydrogen is considered to be little contained. Also, because of a high temperature process, there has been the problem that the substrate used is necessarily limited to glass.